In message , barry.irwin1
writes

(Nothing that can easily be quoted!)

Depends how you define busiest (most frequent? most crowded?) but when
the question has come up here in the past it has often been suggested
that the 73 is the busiest route. However, there are a number of other
routes that operate at a similar 5-minute frequency:

http://www.londonbusroutes.net/details.htm

--
Paul Terry